The
octet rule provides a way to predict what kind of ion an atom will form.
The rule usually applies to the representative elements. The rule has
many exceptions, but is still very useful. The breaks down for elements
in periods 3 through 7.

The
Octet Rule:

Main-group
elements are more stable when they have a complete outer shell
of 8 valence electrons.In other words, the main-group elements
tend to react so they have eight valence electrons.

Calcium
will form a Ca2+ cation. This is based on the fact that
calcium is in Group 2A with two valence electrons.

The electron configuration for the 20 electrons in calcium is 1s22s22p63s23p64s2.

This can be written in a "shorthand" notation as [Ar]4s2.
Here the 18 inner electrons in the 1s22s22p63s23p6 are represented by the symbol [Ar]. The outer electrons
are in the 4s subshell.

The
neutral atom is converted to the +2 ion when the outer electrons are
removed. The octet rule predicts an atom will form an ion with a rare
gas electron configuration. The rare gas in this case is the rare
gas from the previous period.